Glossary of Environmental Terms

 

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This glossary contains terms and definitions extracted from fact sheets and other DTSC publications, most of which were developed to support public participation activities. It is a “working” glossary in that the public participation and technical staff use it as a starting place, but frequently modify the descriptions to match site, facility and community-specific needs. Send your comments to webcoord@dtsc.ca.gov.

Acceptable intake (for subchronic and chronic exposure)
Numbers which describe how toxic a chemical is.   The numbers are derived from animal studies of the relationship between dose and non-cancer effects.  There are two types of acceptable exposure values: one for acute (relatively short-term) and one for chronic (longer-term) exposure.

Acetone              

A widely used, highly voatile solvent.  It is readily absorbed by breathing, ingestion or contact with the skin.  Workers who have inhaled acetone have reported respiratory problems.

See Volatile

Acids

A class of compounds that can be corrosive when concentrated.  Weak acids, such as vinegar and citric acid, are common in foods.  Strong acids, such as muriatic (or hydrochloric), sulfuric and nitric acid have many industrial uses, and can be dangerous to those not familiar to handling them.  Acids are chemical "opposites" to bases, in that they can neutralize each other.

See Alkaline, Base, pH

Action level

A guideline established by environmental protection agencies to identify the concentration of a substance in a particular medium (water, soil, etc.) that may present a health risk when exceeded. If contaminants are found at concentrations above their action levels, measures must be taken to decrease the contamination.

Activated sludge

A term used to describe sludge that contains microorganisms that break down organic contaminants (e.g., benzene) in liquid waste streams to simpler substances such as water and carbon dioxide.  It is also the product formed when raw sewage is mixed with bacteria-laden sludge, then stirred and aerated to destroy organic matter.

Activity (of a radioactive isotope)          

The number of particles or photons ejected from a radioactive substance per unit time.

Acute hazards

Hazards associated with short-term exposure to relatively large amounts of toxic substances.

Adverse health effects

Effects of chemicals or other materials that impair one's health.  They can range from relatively mild temporary conditions such as minor eye or throat irritation, shortness of breath or headaches to permanent and serious conditions such as cancer, birth defects or damage to organs.

Advisory level

The level above which an environmental protection agency suggests it is potentially harmful to be exposed to a contaminant, although no action is mandated.

Aeration

Passing air through a solid or liquid, especially a process that promotes breakdown or movement of contaminants in soil or water by exposing them to air.

Air stripping tower

Air stripping removes volatile organic chemicals (such as solvents) from contaminated water by causing them to evaporate.  Polluted water is sprayed downward through a tower filled with packing materials while air is blown upwards through the tower.  The contaminants evaporate into the air, leaving significantly-reduced pollutant levels in the water.  The air is treated before it is released into the atmosphere. 

See Volatile organic chemicals

Alkaline (synonym basic, caustic)

Having the properties of a base, a pH greater than 7.   Usually used as an adjective, i.e. "alkaline soil". See Acid, Base, pH

Alluvial deposit        

An area of sand, clay or other similar material that has been gradually deposited by moving water, such as along a river bed or shore of a lake.

Alpha particle

A positively-charged particle emitted by radioactive atoms.  Alpha particles travel less than one inch in the air and a thin sheet of paper will stop them.   The main danger from alpha particles lies in ingesting the atoms which emit them.  Body cells next to the atom can then be irradiated over an extended period of time, which may be prolonged if the atoms are taken up in bone, for instance.  See Beta particle, Gamma radiation

Ambient air

Refers to the surrounding air. Generally, ambient air refers to air outside and surrounding an air pollution source location. Often used interchangeably with "outdoor air."

Applicable or Relevant and Appropriate Requirements (ARARs)

Federal or state laws, regulations, standards, criteria or requirements which would apply to the cleanup of hazardous substances at a particular site.

Aqueous

Water-based.

Aquifer

A water-bearing layer of rock or sediment that is capable of yielding useable amounts of water.  Drinking water and irrigation wells draw water from the underlying aquifer.

Arsenic

A gray, brittle and highly poisonous metal.  It is used as an alloy for metals, especially lead and copper, and is used in insecticides and weed killers.  In its inorganic form, it is listed as a cancer-causing chemical under Proposition 65.

Artesian well

A well that flows up like a fountain because of the internal pressure of the aquifer.

See Aquifer.

Asbestos

A general name given a family of naturally occurring fibrous silicate minerals.  Asbestos fibers were used mainly for insulation and as a fire retardant material in ship and building construction and other industries, and in brake shoes and pads for automobiles.  Inhaling asbestos fibers has been shown to result in lung disease (asbestosis) and in lung cancer (mesothelioma).  The risk of developing mesothelioma is significantly enhanced in smokers. 

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Backfill

The word is used in two contexts; to refill an excavated area with uncontaminated soils; and the material used to refill an excavated area.

Background concentration

Represents the average amount of toxic chemicals in the air, water or soil to which people are routinely exposed.  More than half of the background concentration of toxic air in metropolitan areas comes from automobiles, trucks and other vehicles.  The rest comes from industry and business, agricultural, and from the use of paints, solvents and chemicals in the home.

Bases

A class of compounds that are "opposite" to acids, in that they neutralize acids.   Weak bases are used in cooking (baking soda) and cleaners.  Strong bases can be corrosive, or "caustic".  Examples of strong bases that are common around the house are drain cleaners, oven cleaners and other heavy duty cleaning products.  Strong bases can be very dangerous to tissue, especially the eyes and mouth.  See Acid, Alkaline, pH.

Benzene

A petroleum derivative widely used in the chemical industry.  A few uses are: synthesis of rubber, nylon, polystyrene, and pesticides; and production of gasoline.  Benzene is a highly volatile chemical readily absorbed by breathing, ingestion or contact with the skin.  Short-term exposures to high concentrations of benzene may result in death following depression of the central nervous system or fatal disturbances of heart rhythm.  Long-term, low-level exposures to benzene can result in blood disorders such as aplastic anemia and leukemia.  Benzene is listed as a cancer-causing chemical under Proposition 65.

See Volatile.

Berm

A curb, ledge, wall or mound used to prevent the spread of contaminants.  It can be made of various materials, even earth in certain circumstances.  See Alpha particle, Gamma radiation.

Beta particles

Very high-energy particle identical to an electron, emitted by some radioactive elements.  Depending on their energy, they penetrate a few centimeters of tissue.

Bioaccumulation

The process by which  the concentrations of some toxic chemicals gradually increase in living tissue,  such as in plants, fish, or people as they breathe contaminated air, drink contaminated water, or eat contaminated food.

Bioremediation

A process that uses microorganisms to change toxic compounds into non-toxic ones.

Biosolids

Residuals generated by the treatment of sewage, petroleum refining waste and industrial chemical manufacturing wastewater with activated sludge.  See Activated Sludge.

Biota

The animal and plant life of a particular region.

Biotransformation

Transformation of one chemical to others by populations of microorganisms in the soil.

Boring

Usually, a vertical hole drilled into the ground from which soil samples can be collected and analyzed to determine the presence of chemicals and the physical characteristics of the soil.

Borrow pit

An area where soil, sand or gravel has been dug up for use elsewhere.

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Cadmium

Cadmium is a natural element in the earth’s crust, usually found as a mineral combined with other elements such as oxygen.  Because all soils and rocks have some cadmium in them, it is extracted during the production of other metals like zinc, lead and copper.  Cadmium does not corrode easily and has many uses.  In industry and consumer products, it is used for batteries, pigments, metal coatings, and plastics.  Cadmium salts are toxic in higher concentrations.

California Environmental Quality Act (CEQA)

First enacted in 1970 to provide long-term environmental protection, the law requires that governmental decision-makers and public agencies study the significant environmental effects of proposed activities, and that significant avoidable damage be avoided or reduced where feasible. CEQA also requires that the public be told why the lead public agency approved the project as it did, and gives the public a way to challenge the decisions of the agency.

Cancer risk

A number, generally expressed in exponential form (i.e., 1 x 10 -6, which means one in one million), which describes the increased possibility of an individual developing cancer from exposure to toxic materials.  Calculations producing cancer risk numbers are complex and typically include a number of assumptions that tend to cause the final estimated risk number to be conservative.

Cap

A layer, such as clay or a synthetic material, used to prevent rainwater from penetrating the soil and spreading contamination.

Carbamates

A group of insecticides related to carbamic acid. They are primarily used on corn, alfalfa, tobacco, cotton, soybeans, fruits and ornamental plants.

Carbon adsorption

A treatment system in which organic contaminants are removed from groundwater and surface water by forcing it through tanks containing activated carbon, a specially-treated material that retains such compounds.  Activated carbon is also used to purify contaminated air by adsorbing the contaminants as the air passes through it. 

Carbon monoxide

A very poisonous, colorless and odorless gas formed when carbon-containing matter burns incompletely, as in automobile engines or in charcoal grills used indoors without proper ventilation.

Carbon tetrachloride (CCl4)

A colorless, nonflammable toxic liquid that was widely used as a solvent in dry-cleaning and in fire extinguishers.  It is listed as a cancer-causing chemical under Proposition 65.

Catalyst  

A substance that accelerates chemical change yet is not permanently affected by the reaction (e.g., platinum in an automobile catalytic converter helps change carbon monoxide to carbon dioxide).

Catalytic Cracker Unit

In a petroleum refinery, the catalytic cracker unit breaks long petroleum molecules apart, or “cracks” them, during the petroleum refining process. These smaller pieces then come together to form more desirable molecules for gasoline or other products.

Caustic

The common name for sodium hydroxide, a strong base.  Also used as an adjective to describe highly corrosive bases. See Base, pH

Caustic scrubber

An air pollution control device in which acid gases are neutralized by contact with an alkaline solution.

CERCLA

See Comprehensive Environmental Response, Compensation, and Liability Act of 1980.

CEQA

See California Environmental Quality Act.

Chlorinated herbicides

A group of plant-killing chemicals which contain chlorine, used mainly for weed control and defoliation.

Chlorobenzene

A volatile organic compound that is often used as a solvent and in the production of other chemicals.  It is a colorless liquid with an almond-like odor. It is toxic.

See Volatile organic compound (VOCs)

Chloroform

Chloroform was once commonly used as a general anesthetic and as a flavoring agent in toothpastes, mouth wastes and cough syrups.  It is listed as a cancer-causing chemical under Proposition 65.

Chromated copper arsenate

An insecticide and herbicide containing three metals: copper, chromium and arsenic.  This salt is used extensively as a wood preservative in pressure-treating operations.  It is highly toxic and dissolves in water, making it a relatively mobile contaminant in the environment.

Chromium

A hard, brittle, grayish heavy metal used in tanning, in paint formulation, and in plating metal for corrosion protection.  It is toxic at certain levels and, in its hexavalent (versus trivalent) form, chromium is listed as a cancer-causing agent under Proposition 65.

Chronic exposure

Repeated contact with a chemical over a period of time, often involving small amounts of toxic substance.

Class I landfill

A landfill permitted to accept hazardous wastes.

Clean Air Act        

A federal law passed in 1955 and extensively modified in 1970.  It is enforced by the California Air Resources Board and the local air quality management or air pollution control districts, as well as by U.S. EPA nationally.

The Clean Water Act

A federal law of 1977 enforced by U.S. EPA.  A key provision is that “any person responsible for the discharge of a pollutant or pollutants into any waters of the United States from any point source must apply for and obtain a permit.”  This is reflected by the National Pollutant Discharge Elimination System (NPDES), through which the permits are issued by Regional Water Quality Control Boards.  Permits are now being required for stormwater runoff from cities and other locations.

Cleanup process

A comprehensive program for the clean-up (remediation) of a contaminated site.  It involves investigation, analysis, development of a cleanup plan and implementation of that plan.

Combustion gases

Gases produced by burning.  The composition will depend on, among other things, the fuel; the temperature of burning; and whether air, oxygen or another oxidizer is used.  In simple cases the combustion gases are carbon dioxide and water.  In some other cases, nitrogen and sulfur oxides may be produced as well.  Incinerators must be controlled carefully to be sure that they do not emit more than the allowable amounts of more complex, hazardous compounds.  This often requires use of emission-control devices.

Combustible vapor mixture

The composition range over which air containing vapor of an organic compound will burn or even explode when set off by a flame or spark.  Outside that range the reaction does not occur, but the mixture may nevertheless be hazardous because it does not contain enough oxygen to support life, or because the vapor is toxic.

Comprehensive Environmental Response, Compensation and Liability Act of 1980  (CERCLA) (Pronounced SIR-cluh)

Also known as Superfund, this Federal law authorizes U.S. EPA to respond directly to releases of hazardous substances that may endanger public health or the environment.  The Superfund Amendments and Reauthorization Act of 1986 (SARA), amended and reauthorized CERCLA for five years at a total funding level of $8.5 billion.  SARA also strengthened state involvement in the cleanup process, and encouraged the use of new treatment technologies and permanent solutions.  CERCLA has since been extended by other laws.

In particular, SARA Title III is known as the Emergency Planning and Community Right-to-Know Act of 1986.  It requires each state to have an emergency response plan as described, and any company that produces, uses or stores more than certain amounts of listed chemicals must meet emergency planning requirements, including release reporting.

Consent decree

A legal document, approved and issued by a judge, formalizing an agreement between DTSC and the parties potentially responsible for site contamination.  The decree describes cleanup and other actions that the potentially responsible parties are required to perform and the costs incurred by the government that they will reimburse, together with the roles, responsibilities and enforcement options that the government may exercise in the event of non-compliance.  If a settlement between DTSC and a potentially responsible party includes cleanup actions, it must be in the form of a consent decree, which is subject to a public comment period.

Containment

Enclosing or containing hazardous substances in a structure to prevent the migration of contaminants into the environment.

Copper

Distinctively-colored metal used for electric wiring, plumbing, heating and roof and building construction, and in automobile brake linings.  It is known to be toxic at certain levels.

Corrosivity

A characteristic of acidic and basic hazardous wastes.

The characteristic is defined by a waste's pH and its ability to corrode steel. A waste is corrosive if it has a pH less than or equal to 2.0 or greater than or equal to 12.5. See pH, acids, bases

Creosotes

Chemicals used in wood preserving operations that are produced by distilling coal-tar.  They contain polycyclic aromatic hydrocarbons and polynuclear aromatic hydrocarbons (PAHs and PNAs) and so high-level, short-term exposures may cause skin ulcerations.  Creosotes are listed as cancer-causing agents under Proposition 65.

Criteria pollutants

Air pollutants for which standards for safe levels of exposure have been set under the Clean Air Act.  Current criteria pollutants are sulfur dioxide, particulate matter, carbon monoxide, nitrogen oxides, ozone and lead.

Cumulative impact

The term cumulative impact is used in several ways:

as the effect of exposure to more than one compound; as the effect of exposure to emissions from more than one facility; the combined effects of a facility and surrounding facilities or projects on the environment; or some combination of these.

Cyanide

A highly toxic chemical often used in metal finishing or in extraction of precious metal from ore.

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DDT

An environmentally-persistent insecticide banned for most uses by the U.S. EPA in 1972.  It accumulates in fatty tissues of animals, and has lead to serious environmental problems such as thinning of shells of certain birds and their resulting die-off.  It may also have toxic effects on humans upon prolonged exposure.

Degrease

To remove grease from machinery, tools, etc., usually using solvents.  Aqueous (water-based) cleaners are becoming popular and are required in some parts of the state.

Deionized water

Water which has been specifically treated to remove minerals.

De minimis risk

A level of risk that the scientific and regulatory community asserts is too insignificant to regulate.

Department of Toxic Substances Control (DTSC)

A department within the California Environmental Protection Agency charged with the regulation of hazardous waste from generation to final disposal, and for overseeing the investigation and clean-up of hazardous waste sites.

Destruction and removal efficiency (DRE)

A percentage that represents the number of molecules of compound removed or destroyed in an incinerator relative to the number of molecules that entered the incinerator system.  A DRE of 99.99 percent means that 9,999 molecules of a compound are destroyed for every 10,000 molecules that enter the system.  For some compounds a DRE of 99.9999 is required.

Dewater

To remove water from wastes, soils or chemicals.

Diazinon

An organophosphate insecticide.  It is used in agriculture, and for home, lawn and commercial uses.

Dibromochloropropane (DBCP)

An amber-colored liquid used in a agriculture to kill pests in the soil. Inhalation of high concentrations of DBCP causes nausea and irritation of the respiratory tract.  Chronic exposure results in sterility in males.  Although not in use as a pesticide in this country since 1979 (until 1985 in Hawaii), it is found as a contaminant at many hazardous substances sites.  DBCP is listed as a cancer-causing agent under Proposition 65.

Dichlorobenzene (DCB)

A volatile organic compound often used as a deodorizer, and as a moth, mold and mildew killer.  It is a white solid with a strong odor of mothballs.  It is toxic and is listed as a cancer-causing agent under Proposition 65.

1, 1-Dichloroethane

A colorless, oily liquid having an ether-like odor. It is used to make other chemicals and to dissolve other substances such as paint and varnish, and to remove grease. In the past, this chemical was used as a surgical anesthetic, but it is no longer used for this purpose. Because 1,1-dichloroethane evaporates easily into air, it is usually present in the environment as a vapor rather than a liquid.

Dieldrin

Dieldrin is an insecticide that was used on crops like corn and cotton.  U.S. EPA banned its use in 1987.  It is listed as a cancer-causing chemical under Proposition 65.

Dioxins

A group of generally toxic organic compounds that may be formed as a result of incomplete combustion (as may occur in incineration of compounds containing chlorine).  RCRA regulations require a higher destruction and removal efficiency (DRE) for dioxins and related furans (99.9999 percent) burned in incinerators than the DRE required for most other organic compounds (99.99 percent).  They are rapidly absorbed through the skin and gastrointestinal tract and are listed as cancer-causing chemicals under Proposition 65.

Downgradient

The direction in which groundwater flows.

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Effluent

Wastewater, treated or untreated, that flows out of a treatment plant, sewer or industrial outfall.  Generally refers to wastes discharged into surface waters.

Electrostatic precipitator

An air pollution control device that uses electrical charges to remove particulate matter from emission gases.

Emulsifiers

Substances that help in mixing liquids that don't normally mix; e.g., oil and water.

Endosulfan

An insecticide used on vegetable crops, fruits and nuts.

EPCRA (Emergency Planning and Community Right-to-Know Act) (SARA Title III)

See Comprehensive Environmental Response, Compensation, and Liability Act of 1980.

Estuary (adj.: estuarine)

Areas where fresh water from rivers mixes with salt water from nearshore ocean. They include bays, mouths of rivers, salt marshes and lagoons.  These brackish water ecosystems shelter and feed marine life, birds and wildlife.

Ethylene glycol

Used in the manufacture of a wide variety of industrial compounds  and in certain cosmetics.  It is used most commonly as an automobile antifreeze.  It is toxic.

Exposure pathways

Existing or hypothetical routes by which chemicals in soil, water or other media can come in contact with humans, animals or plants.

Extraction wells

Wells that are used primarily to remove contaminated groundwater from the ground.  Water level measurements and water samples can also be collected from extraction wells.

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Fallout

The radioactive dust particles that settle to earth after the denotation of a nuclear device.  It is also used to describe dust particles settling from smoke, etc.

Feasibility study

An evaluation of the alternatives for remediating any identified soil or groundwater contamination.

Filter cake

A mixture of sediments that results from filtering and dewatering of treated wastewater.

Flammables

A class of compounds that ignite easily and burn rapidly. The Department of Transportation requires that Vehicles transporting flammables must have special markings (placards).

Flash point

The lowest temperature at which a liquid generates enough vapor to ignite in air.  If a waste has a flash point of less than 140° F, then it is an ignitable hazardous waste.

Fly ash

Non-combustible residue that results from burning fuels in an incinerator, boiler or furnace.  It can include metal oxides, silicates and sulfur compounds, as well as many other chemical pollutants.  It is fine ash carried along by flue gases that must be captured by some means before it reaches the mouth of the chimney. 

Footprint

The outline of an area within which hazardous substances are suspected or known to exist.

Formaldehyde

A water-soluble gas used widely in the chemical industry and in the construction and building industries, largely in wood products and in foam insulation. It is also used in some deodorizing preparations, in fumigants and as a tissue preservative in laboratories.  Formaldehyde is listed as a cancer-causing agent under Proposition 65.

French drain system   

A pit or trench filled with crushed rock and used to collect and divert stormwater or wastewater.  Most often, perforated piping at the bottom provides easy drainage.

Fugitive emissions

Releases of pollutants to the atmosphere that occur when vapors are vented from containers or tanks where materials are stored.  They can also be caused by spillage during the unloading of vehicles, leaks from pipes and valves, and through equipment operation.

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Gamma radiation

A high-energy photon (ray) emitted from the nucleus of certain radioactive atoms. Gamma rays are the most penetrating of the three common types of radiation (the other two are alpha particles and beta particles) and are best stopped by dense materials such as lead.  See Alpha particle, Beta particle.

Geophysical logging

A general term that encompasses all techniques for determining whether a subsurface geological formation may be sufficiently porous or permeable to serve as an aquifer.  These techniques typically involve lowering a sensing device into a borehole to measure properties of the subsurface formation.

Granular activated carbon (GAC)

A form of crushed and hardened charcoal.  GAC has a strong potential to attract and absorb volatile organic compounds from extracted groundwater and gases.

Groundwater

Water beneath the earth's surface that flows through soil and rock openings, aquifers, and often serves as a primary source of drinking water.

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Half-life

The amount of time that is required for a radioactive substance to lose one-half its activity.  Each radioactive substance has a unique half-life.  It is also used to describe:

the time for a pollutant to lose one half of its concentration, as through biological action; and the time for elimination of one half a total dose of a drug from a body.

Halogens

The family of elements that includes fluorine, chlorine, bromine and iodine. Halogens are very reactive and have many industrial uses.  They are also commonly used in disinfectants and insecticides.  Many hazardous organic chemicals -- such as polychlorinated biphenyls (PCBs), some volatile compounds (VOCs) and dioxins contain halogens, especially chlorine.

Hazardous waste

Waste substances which can pose a substantial or potential hazard to human health or the environment when improperly managed.  Hazardous waste possesses at least one of these four characteristics:  ignitability, corrosivity, reactivity or toxicity; or appears on special U.S. EPA lists.

Health risk/endangerment assessment

A study prepared to assess health and environmental risks due to potential exposure to hazardous substances.

Health-based remediation targets

Levels to which hazardous substances on the site will be cleaned up.  These target levels are health-based, meaning that exposure to the hazardous substances at or below the target is not expected to present a significant health risk.

Heavy metals

A group of elements (such as chromium, lead, copper and zinc) that can be toxic at relatively low concentrations and tend to accumulate in the food chain..

Heptachlor

An organochloride insecticide once  widely used on food crops, especially corn, but has not been in use since 1988.  It is listed as a cancer-causing chemical under Proposition 65.

Horizontal wells

Extraction and monitoring wells are typically drilled vertically.  A horizontal well has the advantage of providing a large area of groundwater capture for a lower overall cost. 

Hot spot criteria

Cleanup levels for small areas on the site that have particularly high concentrations of hazardous substances.

Hydrochloric acid

Clear, colorless and acidic solution of hydrogen chloride in water often used in metal cleaning and electroplating.  Many hazardous wastes contain chlorine compounds which create small amounts of hydrogen chloride when they are burned. This can contribute to the formation of acid rain.  Regulations require that air pollution control equipment remove either 99% of the hydrochloric acid, or that the emissions contain less than four pounds per hour.

Hydrogeology

The geology of groundwater, with particular emphasis on the chemistry and movement of water.

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Ignitability

A characteristic of hazardous waste.  If a liquid (containing less than 24% alcohol) has a flash point less than 140° F, it is a hazardous waste in the United States.

Impoundment

A body of water or sludge confined by a dam, dike, floodgate or other barrier.

In-situ soil aeration

Applying a vacuum to vapor extraction wells to draw air through the soil so that chemicals in the soil are brought to the surface where they can be treated.

Incompatible wastes

Wastes which create a hazard of some form when mixed together.  This could be intense heat or toxic gases, for example.

Indicator chemicals

Chemicals selected from the group of chemicals found at the site and used for a public health evaluation.  They are selected on the basis of toxicity, mobility and persistence, and are thought to be the chemicals of the greatest potential risk.

Interim remedial actions (IRAs)  also known as Interim Remedial Measures

Cleanup actions taken to protect public health and the environment while long-term solutions are being developed.

Irritant

A chemical that can cause temporary irritation at the site of contact.

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Leachate

Typically, water that has come in contact with hazardous wastes.  For example:

Water from rain or other sources that has percolated through a landfill and dissolved or carries various chemicals, and thus could spread contamination.  Current landfills have systems to collect leachate before that can happen.

Lead

A heavy metal present in small amounts everywhere in the human environment.  Lead can get into the body from drinking contaminated water, eating vegetables grown in contaminated soil, or breathing dust when children play or adults work in lead-contaminated areas or eating lead-based paint.  It can cause damage to the nervous system or blood cells.  Children are at highest risk because their bodies are still developing.  Lead and its compounds are listed as a reproductive toxic substance for women and men, and a cancer-causing substance under Proposition 65.

Lead agency

A public agency which has the principal responsibility for ordering and overseeing site investigation and cleanup. 

Lindane

Lindane (gamma hexachlorocyclohexane) is an insecticide, once used on fruit and vegetable crops.  It is still used to treat head and body lice, and scabies.  It is highly toxic to humans, freshwater fish and aquatic life.  It is listed as a cancer-causing chemical under Proposition 65.

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Magnesium oxide

Also known as magnesia, magnesium oxide is used medicinally (“Milk of Magnesia”), industrially and in agricultural soil supplements.  It is also used to enhance biological processes and to cleanup groundwater.

Magnesium

This light metal and its derivatives are used in aerospace alloys, in incendiary devices such as flares, and elsewhere.  When scrap magnesium is thinly shaved or powdered, it is considered to be a hazardous waste, as it ignites easily and burns with an intense, white flame.  It is also a nutritionally essential trace metal.

Malathion

Malathion is an insecticide that, at high doses, affects the human nervous system.

Maximum contaminant level (MCL)

A contaminant level for drinking water, established by the California Department of Health Services, Division of Drinking Water and Environmental Management, or by the U.S. Environmental Protection Agency.  These levels are legally-enforceable standards based on health risk (primary standards) or non-health concerns such as odor or taste (secondary standards).

Mercury

Also known as "quicksilver," this metal is used in the paper pulp and chemical industries, in the manufacture of thermometers, and thermostats, and in fungicides.  Mercury exists in three biologically important forms, elemental, inorganic and organic. It is highly toxic and affects the nervous system, kidneys and other organs.  It also accumulates in animals that are high in the food chain (predators).  Organic mercury compounds are the most toxic, and transformations between the three forms of mercury do occur in nature. 

Methane

An odorless, colorless, flammable gas that is the major constituent of natural gas.  It can be formed from rotting organic matter (i.e., trash in a landfill), and seep up through soils or migrate through underground piping to the surface. It also seeps up through the ground in areas that have shallow petroleum deposits or improperly abandoned oil wells, such as certain areas of the Los Angeles Basin.  If it collects in a closed space and reaches certain concentrations, a spark can cause an explosion.  It can also displace air and cause a suffocation hazard in low, enclosed spaces.  This is one of the reasons landfill gas is collected and burned, sometimes for generation of electricity.

Methyl ethyl ketone (MEK)    

MEK is a flammable solvent that has many industrial uses, primarily in the plastic industry as a solvent.  MEK is also used in the synthetic rubber industry, in the production of paraffin wax, and in household products such as lacquer and varnishes, paint remover, and glues.

Methylene chloride

A colorless liquid that evaporates easily.   It has been used as a metal cleaner, paint thinner, in wood stains, spot removers, fabric protectors, shoe polish and aerosol propellants.  Mild exposure can cause skin and eye irritation

Microgram per gram (mg/g)    

A  measurable unit of concentration for a solid.  A mercury level of 1.0 mg/g means that one microgram (one millionth of a gram) of mercury was detected in one gram of sample.  It is equivalent to one part per million.

Milligram per cubic meter (mg/m3)

A unit of concentration for air contaminants.  A mercury vapor level of 1.0 mg/m3 means that one milligram (one thousandth of a gram) of mercury vapor was detected in each cubic meter of sampled air.

Milligram per kilogram (mg/kg)

A unit of concentration for a solid.  A mercury level of 1.0 mg/kg in fish means that one milligram (one thousandth of a gram) of mercury was found in each kilogram of sampled fish.  (A kilogram is 1,000 grams or approximately 2.2 pounds).  Also equals one part per million.

Migration

The movement of chemical contaminants through soils or groundwater.

Mitigation

Actions taken to improve site conditions by limiting, reducing or controlling hazards and contamination sources.

Monitoring wells

Specially-constructed wells used exclusively for testing water quality.

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National Pollutant Discharge Elimination System (NPDES)

A system under the federal Clean Water Act that requires a permit for the discharge of pollutants to surface waters of the United States.  In California, NPDES permits are obtained from the Regional Water Quality Control Board.

National Priorities List (NPL)

U.S. EPA's list of the top priority hazardous waste sites in the country that are subject to the Superfund program.

Negative Declaration

A California Environmental Quality Act document issued by the lead regulatory agency when the initial environmental study reveals no substantial evidence that the proposed project will have a significant adverse effect on the environment, or when any significant effects would be avoided or mitigated by revisions agreed to by the applicant.

Neutrals

Organic compounds that have a relatively neutral pH (are neither acid nor base), complex structure and, due to their carbon bases, are easily absorbed into the environment.  Naphthalene, pyrene and trichlorobenzene are examples of neutrals.

Nickel

A metal used in alloys to provide corrosion and heat resistance for products in the iron, steel and aerospace industries.  Nickel is used as a catalyst in the chemical industry.  It is toxic and, in some forms, is listed as a cancer-causing agent under Proposition 65.

Nitrate

Formed when ammonia is degraded by microorganisms in soil or groundwater.  This compound is usually associated with fertilizers. 

Nitroaromatics

Common components of explosive materials, which will explode if activated by very high temperatures or shocks.  2,4,6-trinitrotoluene (TNT) is a nitroaromatic.  Some are listed as cancer-causing chemicals under Proposition 65.

Non-attainment pollutants

See “Criteria pollutants”.  If any of the criteria pollutants exceed established health-based levels in a given air basin, they are identified as “non-attainment pollutants”.

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Operation Plan

A document submitted  to DTSC that gives details about how a permitted hazardous waste  facility is built, a detailed description of the hazardous waste operations, the plan to be used in case of emergency, and other plans.  A DTSC facility permit requires that the reviewed and approved Operations Plan be followed.  It is sometimes referred to as the "Part B" of the hazardous waste facility permit.

Organochlorides

A group of organic (carbon-containing) insecticides that also contain chlorine.  These chemicals tend not to break down easily in the environment.  DDT, Toxaphene and Endosulfan are all organochlorides.

Organophosphate

A group of organic (carbon-containing) insecticides that also contain phosphorus.  Although they do not have a long life, some can be very toxic when first applied.  Malathion and Parathion are organophosphates.  Malathion is mildly toxic, and parathion is extremely toxic.

Overpacking

Process used for isolating waste by jacketing or encapsulating waste-holding containers to prevent further spread or leakage of contaminating materials.  Leaking drums may be contained within oversized ones as an interim measure prior to removal and final disposal.

Oxidizers  

A group of chemicals that are very reactive, often but not always supplying oxygen to a reaction.  Some oxidation reactions can release large amounts of heat and gases, and, under the right conditions, cause an explosion.  Others can cause rapid corrosion of metal, damage to tissue, burns and other serious effects.  Examples of oxidizers include chlorine gas, nitric acid, sodium perchlorate, and ammonium nitrate.

Ozone and ozonation

Ozone is a reactive form of oxygen (O3) that reacts with volatile organic compounds (VOCs) to change them into chemicals which pose no potential threat to human health, by breaking them down to form carbon dioxide and water.  This is done with an ozonation unit

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Parathion and Methylparathion

Parathion and Methylparathion are toxic insecticides.

Particulates

Small solid or liquid particles, especially those in the emission gases of incinerators, boilers, industrial furnaces or in exhaust from diesel and gasoline engines. Particles below 10 microns (10 one-millionths of a meter, 0.0004 inch) in diameter are considered potential health risks because, when inhaled, they are taken deep into the lungs.  Regulations require that an incinerator emit no more than 180 milligrams of total particulates per dry standard cubic meter per minute.

Parts per million (ppm)

A measuring unit for the concentration of one material in another.  When looking at contamination of water and soil, the toxins are often measured in parts per million.  One part per million is equal to one thousandth of a gram of substance in one thousand grams of material. One part per million would be equivalent to one drop of water in twenty gallons. See milligrams per kilogram.

Parts per billion (ppb)

A unit of measure used to describe levels or concentrations of contamination.   A measure of concentration, equaling 0.0000001 percent.  For example, One part per billion is the equivalent of one drop of impurity in 500 barrels of water.  Most drinking water standards are ppb concentrations. 

Pentachlorophenol (PCP)

A petroleum-based chemical that is used as a wood preservative because it kills fungus and termites.  It is toxic listed as a cancer-causing chemical under Proposition 65.

Perched groundwater

Water that accumulates beneath the earth's surface but above the main water bearing zone (aquifer).  Typically, perched groundwater occurs when a limited zone (or lens) of harder, less permeable soil is "perched" in otherwise porous soils.  Rainwater moving downward through the soil stops at the lens, flows along it, then seeps downward toward the aquifer.

Perchloroethylene (PCE)

A volatile organic compound used primarily as a dry-cleaning agent.  It is often referred to as "perc."  It is toxic and listed as a cancer-causing chemical under Proposition 65.

Percolation

The downward flow or filtering of water or other liquids through subsurface rock or soil layers, usually continuing to groundwater.

Pesticide

A general term for insecticides, herbicides and fungicides.  Insecticides kill or prevent the growth of insects.  Herbicides control or destroy plants.  Fungicides control or destroy fungi.  Some pesticides can accumulate in the food chain and contaminate the environment. 

Petrex Method                         

A method for collecting vapor samples from surface soil.

Petrochemicals

Chemical substances produced from petroleum in refinery operations.  Many are hazardous.

pH

A convenient way of describing the strength of an acidic or basic aqueous solution.  The values range from 0 to 14, with a pH of 7 corresponding to neutral.  As the pH number becomes smaller by one unit, the acidity increases by a factor of 10 (for 2 units, it changes by 100, and so on). Likewise, as the pH number increases by one unit, the alkalinity (basic property) increases by a factor of 10, etc.; tap water may lie in a region from above 6 to below 8.  Strongly acidic waste solutions (pH less than 2) and strongly basic ones (pH greater than 12.5) are defined as hazardous wastes because of their corrosive effect on metals and on skin.  See Acid, Alkaline, Base.

Phenols

Organic compounds used in plastics manufacturing, tanning, and textile, dye and resin manufacturing. They are by-products of petroleum refining.  In general, they are highly toxic.

Piezometers

Small-diameter wells used to measure groundwater levels.

Pilot study

A study of a possible cleanup alternative during the Feasibility Study for a specific site.  It is used to gather data necessary for the final selection of the cleanup method.

Plume

A body of contaminated groundwater flowing from a specific source.  The movement of the groundwater is influenced by such factors as local groundwater flow patterns, the character of the aquifer in which the groundwater is contained, and the density of contaminants.  A plume may also be a cloud of smoke or vapor.  It defines the area where exposure would be dangerous.

Polychlorinated biphenyls (PCBs)

A group of toxic chemicals used for a variety of purposes including electrical applications, carbonless copy paper, adhesives, hydraulic fluids, and caulking compounds.  PCBs do not breakdown easily and are listed as cancer-causing agents under Proposition 65.

Polynuclear aromatic hydrocarbons (PAHs or PNAs)

PNAs or Polynuclear Aromatic Hydrocarbons, are natural constituents of crude oil, and also may be formed when organic materials such as coal, oil, fuel, wood or even foods are not completely burned.  PNAs are also found in lampblack, a by-product of the historic gas manufacturing process.  PNAs are found in a wide variety of other materials, including diesel exhaust, roofing tars, asphalt, fireplace smoke and soot, cigarettes, petroleum products, some foods, and even some shampoos.  PNAs tend to stick to soil and do not easily dissolve in water, and generally do not move in the environment.  The test method used to analyze for PNAs detects seventeen different compounds.  Of the seventeen, seven are suspected of causing cancer in humans.         

Polyvinyl chloride (PVC)

A plastic made from the gaseous chemical vinyl chloride.  PVC is used to make pipes, records, raincoats and floor titles.  It produces hydrochloric acid when burned.  Health risks from high concentrations of vinyl chloride  (not the polymer) include liver cancer and lung cancer, as well as cancer of the lymphatic and nervous systems.  Vinyl chloride (not the polymer) is listed as a cancer-causing chemical under Proposition 65.

Potentially Responsible Party (PRP)

An individual, company or government body identified as potentially liable for a release of hazardous substances to the environment.  By federal law, such parties may include generators, transporters, storers and disposers of hazardous waste, as well as present and past site owners and operators.

Pretreatment unit

A wastewater treatment unit that is designed to treat wastewater that does not meet the sewage discharge standards so that it meets or exceeds those standards.  Pretreatment units usually require a permit from a local agency.

Principal organic hazardous constituents (POHCs)

Specific hazardous compounds monitored during an incinerator, boiler or industrial furnace trial burn.  They are selected on the basis of their high concentration in the waste feed and the difficulty of burning them.

Proprietary information (trade secret)

The Department will classify information as proprietary provided the owner demonstrates the following:  the business has asserted a business confidentiality claim; the business has shown it has taken reasonable measures to protect the confidentiality of the information both within the company and from outside entities; the information is not, and has not been reasonably obtainable without the business' consent; no statute specifically requires disclosure of the information ; and either the business has shown that disclosure of the information is likely to cause substantial harm to its competitive position, or the information is voluntarily submitted and its disclosure would likely impair the government's ability to obtain necessary information in the future.

Public participation plan

A document approved by DTSC that is designed to determine a community's informational needs and to provide a program for public involvement during facility permitting, site investigation and cleanup, or other similar activities.

Pump test

A field test by which a well is pumped for a period of time and data are collected for use in assessing characteristics of subsurface water-bearing zones, or aquifers.

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Quench tower
A gas cooling and pollution control device in which heated gases are showered with water.  Gases are cooled and particulates "drop out" of the gases.  They can generate a waste called "quench tower drop-out".

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Radiation
The process of emitting energy in the form of energetic particles (such as alpha particles or gamma radiation), light or heat.  It also refers to that which is emitted.

Radionuclides
Radioactive elements, which may be naturally-occurring or synthetic.  They emit various types of energetic radiation --- alpha and beta particles and gamma radiation.  Their half-lives range from a minute fraction of a second to many thousand years.  Certain radionuclides have valuable medical and industrial uses.  One is used in home smoke detectors at an amount that can cause no harmful effects.

Radium
A radioactive element with a half-life of 1,600 years that emits alpha particles as it is transformed into radon.  In the past, radium was mixed with special paints to make watch faces and instrument dials glow in the dark.

Radon
A gaseous, radioactive alpha particle-emitting element with a half-life of about four days.  Radon exists naturally in many locations, and may present a serious health risk when it accumulates in basements or crawl spaces beneath homes.

RCRA   (pronounced “REC-ra”)
See Resource Conservation and Recovery Act

Reactive
A class of compounds which are normally unstable and readily undergo violent change, react violently with water, can produce toxic gases with water, or possess other similar properties. Reactivity is one characteristic that can make a waste hazardous.

Regional Water Quality Control Board (RWQCB)
Agencies that maintain water quality standards for areas within their jurisdictions and enforce state water quality laws.  

Remedial Action Plan (RAP)
A plan that outlines a specific program leading to the remediation of a contaminated site.  Once the Draft Remedial Action Plan is prepared, and approved by DTSC a public meeting is held and comments from the public are solicited for a period of not less then 30 days.  After the public comment period has ended and the comments have been responded to in writing, DTSC may modify the Draft Plan on the basis of those comments before it approves the final remedy for the site (the Final RAP).

Remedial Investigation/Feasibility Study (RI/FS)
A series of investigations and studies to identify the types and extent of chemicals of concern at the site and to determine cleanup criteria (Remedial Investigation), and to provide an evaluation of the alternatives for remediating any identified soil or groundwater problems (Feasibility Study).

Remediation
Cleanup of a site to levels determined to be health-protective for its intended use.

Resource Conservation and Recovery Act (RCRA)
A 1976 amendment to the first federal solid waste legislation, the Solid Waste Disposal Act of 1965. In RCRA, Congress established initial directives and guidelines for U.S. EPA to regulate and managesolid waste, including hazardous waste.   RCRA established a regulatory system to track hazardous substances from the time of generation to final disposal. The law requires safe and secure procedures to be used in treating, transporting, storing and disposing of hazardous wastes.  RCRA was designed to prevent new, uncontrolled hazardous waste sites.

Responsible party
An individual or corporate entity considered legally liable for contamination found at a property and, therefore, responsible for cleanup of the site.

Risk assessment
A risk assessment looks at the chemicals detected at a site, the frequency and concentration of detected chemicals, the toxicity of the chemicals and how people can be exposed, and for how long.  Routes of exposure to people are generally through ingestion, such as eating, contact with the skin, or inhalation.  The most significant potential routes of exposure are trough ingestion and contact with the skin.  Based on the standard risk assessment guidelines established for use nationwide by U.S. EPA, exposures for an on-site resident are generally assumed to e daily contact over a 30-year period starting with children ages 0-6, and continuing from 6-30 years.  The health risk assessment cannot predict health effects; it only describes the increased possibility of adverse health effects, based on the best scientific information available.

Rotary kiln
An incinerator with a rotating combustion chamber.  The rotation helps mix the wastes and promotes more complete burning.  They can accept gases, liquids, sludges, tars and solids, either separately or together, in bulk or in containers.

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Sanitary landfill
A landfill which does not take hazardous waste, often called a "garbage dump."  It must be covered with dirt each day to maintain sanitary conditions.  The Integrated Waste Management Board regulates these facilities.

SARA
See Superfund Amendments and Reauthorization Act of 1986.  See Comprehensive Environmental Response, Compensation and Liability Act of 1980.

Secondary containment
A structure designed to capture spills or leaks, as from a container or tank.  For containers and aboveground tanks, it is usually a bermed area of coated concrete.  For underground tanks, it may be a second, outer, wall or a vault.  Construction of such containment must meet certain requirements, and periodic inspections are required.

Sediment
The soil, sand and minerals at the bottom of surface waters, such as streams, lakes and rivers. Sediments capture or adsorb contaminants.  The term may also refer to solids that settle out of any liquid.

Seismic stability
The likelihood that soils will stay in place during an earthquake.

Selenium
This metal is a nutritionally essential trace element that is toxic at higher doses.  High levels of selenium have been shown to cause reproductive failure and birth defects in birds.

Semivolatile organic compounds
Compounds that evaporate slowly at normal temperatures.

Silver
Silver is a metal used in the manufacture of photographic plates, cutlery and jewelry.  Silver nitrate is used in an array of industrial chemical processes.  It is toxic.

Silvex
A chlorinated herbicide.

Sinkhole
A depression formed when the surface collapses into a cavern.

Site mitigation process
The regulatory and technical process by which hazardous waste sites are identified and investigated, and cleanup alternatives are developed, analyzed, decided upon and applied.

Slurry wall
Barriers used to contain the flow of contaminated groundwater or subsurface liquids.  Slurry walls are constructed by digging a trench around a contaminated area and filling the trench with a material that tends not to allow water to pass through it.  The groundwater or contaminated liquids trapped within the area surrounded by the slurry wall can be extracted and treated.

Soil borings
Soil samples taken by drilling a hole in the ground.

Soil gas survey
Soil gas or (soil vapor)  is air existing in void spaces in the soil between the groundwater and the ground surface. These gases may include vapor of hazardous chemicals as well as air and water vapor.  A soil-gas survey involves collecting and analyzing soil-gas samples to determine the presence of chemicals and to help map the spread of contaminants within soil.

Soil vapor extraction
A process in which chemical vapors are extracted from the soil by applying a vacuum to wells.

Solid waste management units   (SWMUs)
Any unit at a hazardous waste facility from which hazardous chemicals might migrate, whether or not they were intended for waste management.   They include such things as containers, tanks, landfills among others.

Solidification
Mixing additives, such as fly ash or cement, with soil containing hazardous chemicals, especially metals, to make it more stable.  This process lessens the risk of exposure to the hazardous chemicals by making it less likely that those chemicals will move into and through surface or groundwater.

Soluble Threshold Limit Concentration
The limit concentration for toxic materials in a sample that has been subjected to the California Waste Extraction Test (WET), a state test for the toxicity characteristic that is designed to subject a waste sample to simulated conditions of a municipal waste landfill.  If the concentration of a toxic substance in the special extract of the waste exceeds this value, the waste is classified as hazardous in California.  This is distinct from the Total Threshold Limit Concentration (TTLC).  The California Waste Extraction Test procedure is more stringent than the federal Toxicity Characteristic Leaching Procedure (TCLP).

Solvent
A liquid capable of dissolving another substance to form a solution.  Water is sometimes called “the universal solvent” because it dissolves so many things, although often to only a very small extent. Organic solvents are used in paints, varnishes, lacquers, industrial cleaners and printing inks, for example.  The use of such solvents in coatings and cleaners has declined over the last several years, because the most common ones are toxic, contribute to air pollution and may be fire hazards.

Stabilization
Changing active organic matter in sludge into inert, harmless material.  The term also refers to physical activities such as compacting and capping at sites that limits the further spread of contamination without actual reduction of toxicity.

State action level (SAL)
The maximum concentration of a contaminant in drinking water that The California Department of Health Services considers to be safe to drink.   Drinking Water Action Levels (ALs) are health-based advisory levels established by the Department of Health Services (DHS) for chemicals for which primary maximum contaminant levels (MCLs) have not been adopted.  There are currently 36 ALs.  ALs are usually expressed in parts per billion (ppb) or parts per million (ppm).  Drinking water with concentrations of impurities greater than the state action level must be treated to reduce or remove the impurities.

Static stability
The likelihood that soils at rest will remain at rest.

Still bottoms
Residues left over from the process of recovering spent solvents in a distillation unit.

STLC
See Soluble Threshold Limit Concentration

Subsidence
Sinking or settling of soils so that the surface is disrupted, creating a shallow hole.

Suggested No Adverse Response Level (SNARL)
Drinking water standards established by the U.S. EPA, but not enforceable by law. SNARLs suggest the level of a containment in drinking water at which adverse health effects would not be anticipated (with a margin of safety).

Sump
A pit or tank that catches liquid runoff for drainage or disposal.

Superfund
See Comprehensive Environmental Response, Compensation and Liability Act of 1980 (CERCLA).

Superfund Amendments and Reauthorization Act (SARA)
Modifications to CERCLA enacted on October 17, 1986.   See CERCLA.

Surge tanks
A tank used to absorb irregularities in flow of liquids, including liquid waste materials, so that the flow out of the tank is constant.

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Tailings or Mine Tailings
Crushed waste rock deposited on the ground during mining and ore processing, including some of the rock in which the ore is found.  Unless they are handled carefully, they frequently release contaminants.  As they age under the effects of air, rainfall and bacteria, some oxidize to produce new toxic materials, such as sulfuric acid, that can leach out and poison streams, rivers and lakes.

TCLP (pronounced “TEE-clip”)
See Toxicity Characteristic Leaching Procedure.

Tetrachloroethylene (TCE)
Volatile organic compound that is commonly used as an industrial degreasing solvent.   TCE affects the central nervous system and is listed as a cancer-causing chemical under Proposition 65.

Tetrachlorophenol (TCP)
Tetrachlorophenol is a toxic fungicide.

Threshold Limit Value (TLV)
Public health exposure level set by the National Institute for Occupational Safety and Health for worker safety.   It is the level above which a worker should not be exposed for the course of an eight-hour day, due to possible adverse health effects.

Toluene
A toxic volatile organic compound often used as an industrial solvent.

Total Threshold Limit Concentration (TTLC)
A test for the toxicity characteristic:   If the total concentration of a toxic substance in a waste is greater than this value, the waste is classified as hazardous in California.  This is distinct from the Soluble Threshold Limit Concentration, or STLC, which is concerned with only the soluble concentration.

Toxaphene
A chlorinated pesticide insecticide that was widely used to control pests on cotton and other crops until 1982, when it was banned for most uses.  (In 1990, banned for all uses.)  It was also used to kill unwanted fish in lakes.  It is toxic to fresh-water and marine aquatic life and is listed as a cancer-causing chemical under Proposition 65.

Toxic Substances Control Act (TSCA)
A federal law of 1976 to regulate chemical substances or mixtures that may present an unreasonable risk of injury to health or the environment.

Toxicity
Ability to harm human health or environment, such as injury, death or cancer.  One of the criteria that is used to determine whether a waste is a hazardous waste (the "Toxicity Characteristic").

Toxicity Characteristic Leaching Procedure (TCLP)
A federal test for the Toxicity Characteristic (TC).   If the concentration of a toxic substance in a special extract of a waste exceeds the TC value, the waste is classified as hazardous in the United States (a “RCRA waste”).  The extraction procedure is different from that of the California Waste Extraction Test (WET).

Trial burn
A test of incinerators or boilers and industrial furnaces in which emissions are monitored for the presence of specific substances, such as organic compounds, particulates, metals and hydrogen chloride (all specified by agency permits).

Trichloroethane
Trichloroethane (1,1,1-TCA; methylchloroform) is used as a cleaning agent for metals and plastics. It is toxic.

Trichloroethylene (TCE)
A volatile organic compound that is often used an industrial degreasing solvent.   It is toxic and is listed as a cancer-causing chemical under Proposition 65.

Tritium
A radioactive form of hydrogen with a half-life of 12 years. It emits beta particles. It is used to mark chemical compounds so that the structure or chemical activity can be determined.  Also used in nuclear weapons research and construction.  Small amounts of tritium occur naturally, and some exists as a by-product of previous nuclear testing and nuclear reactor operations.

TSCA (pronounced “TOSS-kah”)
See Toxic Substances Control Act.

TTLC

See Total Threshold Limit Concentration.

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Unsaturated zone
Underground soil and gravel that could contain groundwater, but lies above the aquifier.  This is in contrast to a saturated zone, where the space between soil particles is filled with water.

Upgradient
The direction from which water flows in an aquifer.  In particular, areas that are higher than contaminated areas and, therefore, are not prone to contamination by the movement of polluted groundwater.

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Vanadium
A toxic metal that is both mined and is a by-product of petroleum refining.  Compounds of vanadium are used in the steel industry, as a catalyst in the chemical industry, in photography and in insecticides.

Video logging
A method for close-up inspection of the interior of a well or pipe by means of a color camera that can view the well casing and screen at 90 degrees to the well's axis.

Vinyl chloride
Vinyl chloride is widely used in the plastics industry in creating polyvinyl chloride (PVC).  It is listed as a cancer-causing agent under Proposition 65.

Viscosity
A measure of the ease with which a liquid can be poured or stirred.  The higher the viscosity, the less easily a liquid pours.

Void space
The space in a tank between the top of a tank and the liquid level.  If the tank is used to store combustible liquids that easily evaporate, this space can fill with vapors which may reach explosive levels.

Volatile
Describes substances that readily evaporate at normal temperatures and pressures.

Volatile organic compounds (VOCs)
Organic liquids, including many common solvents, which readily evaporate at temperatures normally found at ground surface and at shallow depths.   They take part in atmospheric photochemical (sun-driven) reactions to produce smog.

Volatilization rate
The rate at which a chemical changes from a liquid to gas.  It is also known as "air flux."

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Waste feed
The flow of wastes into an incinerator, boiler or industrial furnace.  The waste feed can vary from continuous to intermittent (batch) flows.

Water table
In a shallow aquifer, a water table is the depth at which free water is first encountered in a monitoring well.

Watershed

The land area from which water drains to a given point.

Wetland
An area that is regularly saturated by surface or groundwater and, under normal circumstances, capable of supporting vegetation typically adapted for life in saturated soil conditions; they are critical to sustaining many species of fish and wildlife, including native and migratory birds. They include swamps, marshes, and bogs, and may be either coastal or inland.  Coastal wetlands are brackish (have a certain mixture of salt).

Work plan
The site work plan describes the technical activities to be conducted during the various phases of a remediation project.

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Xylene
An aromatic hydrocarbon used in gasoline, paints, lacquers, pesticides, gums, resins and adhesives.  It is toxic and flammable.

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Zinc
A metal used for auto parts, for galvanizing, and in production of brasses and dry cell batteries.  It is nutritionally essential but toxic at higher levels.